Description from the syntypes. Stems at least 180 mm high, thick and dark brown, basally polysiphonic. Stem divided into short and wide internodes, usually with two apophyses, but internodes with four or six apophyses also present. Nodes sometimes obscure and often incomplete. Cauline apophyses strongly directed upwards, frequently followed by a few short athecate internodes. Hydrocladia initially also strongly directed upwards, curving outward later.

Cauline apophyses with four to six axillary nematophores, each emerging through a perisarc hole ( Fig. 4BView FIGURE 4), occasionally with extremely short collar-shaped nematotheca, and two extra nematophores, each emerging through a strongly marked ‘mamelon’ ( Fig. 4BView FIGURE 4).

Remarks. We observed a maximum of five axillary nematophores in the examined cauline apophyses of the type material, although one or two of the central nematophores are frequently distinctly larger and have a central constriction, which may indicate that they result from the fusion of two.

The description of Oswaldella terranovae was based on a series of ten microscope slides ( BMNH 1929.10.28.171) belonging to the material from the British Antarctic Terra Nova Expedition assigned to Oswaldella antarctica ( Jäderholm, 1904) by Totton (1930). According to Peña Cantero & Vervoort (1996) the cauline apophyses were provided with two axillary nematophores emerging through perisarc holes and two nematophores emerging through ‘mamelons’ placed on the upper part and near the distal end of the apophyses. The SEM study carried out here has shown that O. terranovae has 4 to perhaps 6 axillary nematophores (we observed up to five, but it is likely that other cauline apophyses have six). The fact that the cauline apophyses lie closely on the cauline internodes makes it very difficult to observe the axillary nematophores, much more when the material available is in slides, as when Peña Cantero & Vervoort (1996) described the species.

It has always been difficult to distinguish between Oswaldella stepanjantsae and O. terranovae because of their similarity in colony structure and in the shape and size of their hydrothecae. Peña Cantero & Vervoort (1996: 138) already indicated that O. stepanjantsae ‘resembles O. terranovae in almost all its characteristics’. According to these authors both have ‘robust colonies, stems of lower order, the stems divided into internodes, usually short athecate internodes following the cauline apophyses and hydrocladia of the third order’ (see also Peña Cantero et al. 1997). Peña Cantero et al. (1997) highlighted a few differences between the species, the most important of which was the apparent presence of two axillary nematophores in O. terranovae and four in O. stepanjantsae. Later, however, it was demonstrated that O. stepanjantsae can have up to six axillary nematophores (cf. Peña Cantero & Vervoort 2004; González Molinero & Peña Cantero 2015)] and, as it has been shown in this study, O. terranovae actually has up to five, probably six. As a result, this character can no longer be used to distinguish them.

Even though the gonothecae in the type material of O. terranovae are immature [see fig. 1h in Peña Cantero & Vervoort (1996)], mature gonothecae have been described in material identified as O. stepanjantsae [see for example, fig. 11G – H in Peña Cantero et al. (1997)].

Distribution. Circum-Antarctic. Oswaldella terranovae had only been reported off Cape Adare (Totton 1930; Peña Cantero & Vervoort 1996), but O. stepanjantsae has been considered to have a circum-Antarctic distribution (Peña Cantero & Vervoort 1998).